'It is 23 December 1861. A group of deserters from the American Civil War are attacking a farmhouse near the Potomac. They have killed William Watkins, the farmer, but his daughter is alone in the farmhouse. She has killed at least one attacker due to the rifle skills she was taught by her husband who is fighting alongside General Stuart in Centreville. While defending the back of the house she shoots another man who emerges from the darkness. It is Turlough, and he is wounded in the leg. The Doctor appears too and implores her not to shoot. He calls her by her name, Mrs Elkins, and tells her she must leave the farm. He adds that he has been sent by her husband, Samuel. Charlotte refuses and the Doctor and Turlough return to the TARDIS. Turlough is keen to leave her to her fate but the Doctor insists that they have made a promise to her husband. Lucius Casler, the leader of the attackers, orders his men to roll a burning cart into the farmhouse. When one of his men objects, thinking they have come to loot the house, Casler shoots him dead. Before Casler has time to carry out his attack Samuel Elkins appears and Casler stumbles into the flaming cart. His uniform catches fire and he is killed when his cartridge belt explodes. The rest of his men disappear into the night. Later, Turlough and the Doctor watch the reunion between Samuel and Charlotte. Turlough suggests that the Doctor ought to take Elkins back but the Doctor admits that any damage has already been done when he took Elkins out of his own time and he says that he will give the couple a night together as an early Christmas present. The next morning the Doctor, Turlough and Elkins leave after placing Watkins and the three dead deserters ready for the undertaker. Charlotte is not initially alarmed when the undertaker arrives with a single coffin. Four days earlier Casler murders Elkins on the battlefield and takes a photograph of Charlotte from the dying officer. As Casler looks at the portrait — his Christmas gift, he thinks — he sees, but does not heed, the Doctor emerging onto the battlefield and listening to the last words of the dying Elkins.'
[The Doctor is the Fifth Doctor.]
Source: drwhoguide.com (http://www.drwhoguide.com/whotrip18.htm). Sighted 27/5/11